THIS INVENTION relates to improvements in or relating to an a.c. current distribution system and more particularly relates to an a.c. current distribution system for minimising the electric field along the current distribution system.
A typical a.c. voltage distribution system is shown in FIG. 1 of the accompanying drawings. The a.c. voltage distribution system comprises first and second voltage generators which generate, respectively, a.c. voltages VA and VB, VA being equal to and 180xc2x0 out of phase with VB such that VA=VB. The two voltages are fed down a power bus comprising a pair of conductive tracks which run parallel to one another and are separated from one another. As seen in FIG. 1, various impedance loads may be connected to the tracks along the length of the tracks. Such a voltage distribution system is characterised by the sum of the currents in the adjacent tracks at any one instant in a specific locality along the tracks being zero thereby resulting in a low magnetic field (H-field). Similarly, the sum of the voltages in the adjacent tracks at any instant in a specific locality along the tracks are also zero. This results in a low electric field (E-field).
In some applications, it is preferable to use an a.c. current distribution system rather than an a.c. voltage distribution system such as a current loop system. An example of such a current distribution system is shown in FIG. 2 of the accompanying drawings.
A typical a.c. current distribution system Comprises two a.c. current generators which generate, respectively, currents I and {overscore (I)} at voltages V1 and V2, where V2=V1. The current generators are regulated to be constant and precisely antiphase with one another, although the amplitude of the current need not be precisely regulated. The currents are fed to a current loop comprising a pair of conductive tracks which run parallel to one another and are separated from one another. Any impedance loads to be powered from the current loop system are connected in series to one or other of the tracks. At any instant, the sum of the currents in a specific locality along the lengths of the tracks is zero. This results in a low magnetic field. However, in contrast to the a.c. voltage distribution system, the sum of the voltages at any instant along the tracks in a specific locality is not zero and, in fact, increases along the length of the tracks depending upon the number of loads connected in series along the tracks. This results in a worsening electric field along the length of the tracks. For example, in the locality immediately between the current generators and a first load, the sum of the voltages is zero at any one instant. In the locality immediately after the first load and before the second load, the sum of the voltages is: xcexa3V=V1+V1xe2x88x92VLoad. Further, at the tip of the loop, the sum of the voltages, xcexa3V, equals 2V1. The increase in the sum of the voltages, xcexa3V, from 0 to 2V1 results in a worsening electric field along the length of the track.
It is an object of the present invention to provide an a.c. current distribution system which does not suffer from the above-mentioned disadvantages.
Accordingly, one aspect of the present invention provides an a.c. current distribution system fed by a current source for providing electrical power to a load, the current distribution system comprising a first and a second conductive means which run parallel to one another, which are connectable, respectively, at one end to the current source and which are connected together at the other end to form a current loop, and coupling means to couple substantially one half of the load in series at a first position along the first conductive means and to couple substantially the other half of the load in series at a second position along the second conductive means, the first and second positions being substantially adjacent one another.
Another aspect of the present invention provides a method of reducing the electric field in a current distribution system comprising the steps of coupling a load to be powered by a current source feeding the current distribution system to a first and second conductive means which run parallel to one another, which are connectable, respectively, at one end to the current source and which are connected together at the other end to form a current loop, wherein substantially one half of the load is coupled in series at a first position along the first conductive means and substantially the other half of the load is coupled in series at a second position along the second conductive means, the first and second positions being substantially adjacent one another such that the sum of the voltages on the conductive means in the same locality at any one instant is zero.
Conveniently, the load comprises two distinct half loads, each of which is ohmically connected in series to the respective conductive means.
Preferably, the load is inductively coupled to the respective conductive means by a transformer.
Advantageously, the load is ohmically connected across the terminals of one or more secondary windings of the transformer and the coupling means comprises a pair of substantially identical primary windings of the transformer, each of which is ohmically connected in series to the respective conductive means, the voltage drops across the primary windings being substantially identical, such that the load is split substantially equally between the two primary windings.
In order that the present invention may be more readily understood, embodiments thereof will now be described, by way of example, with reference to the accompanying drawings, in which: